In recent years, a projection apparatus that projects illumination light modulated by an image display element, such as a liquid crystal and a digital micromirror device (hereinafter also referred to as “DMD”), on a surface to be projected, such as a screen, has become widely used as a projector for home theaters.
This kind of projection apparatus usually includes an image display element for modulating illumination light, an illumination optical system for irradiating the image display element with the illumination light, and a projection optical system for projecting the light modulated by the image display element onto a projection surface.
In this projection apparatus, brightness and contrast of a projection image, which is projected on a screen, are affected by the state of an aperture diaphragm of the above-mentioned projection optical system. To be more specific, when an aperture diameter of the aperture diaphragm provided in the projection optical system is larger, the projection image becomes brighter, but the contrast is lower. Conversely, when the aperture diameter of the aperture diaphragm is smaller, the projection image becomes darker, but the contrast is higher.
Therefore, a projection apparatus whose projection optical system is provided with a variable-aperture diaphragm whose aperture diameter can be adjusted so that the brightness and the contrast of the projection image can be adjusted depending on the image to be displayed (for example, refer to Laid-Open Japanese Patent Application Publication No. H05-188345).
On the other hand, a discharge lamp with a large amount of light per unit area is preferably used as a light source for the above-mentioned illumination optical system. However, a parallel light, which is generated by collimating the light from the discharge lamp, does not have an even spatial light intensity distribution, and the light intensity in the area near an optical axis is smaller than in the circumferential area.
Therefore, an illumination optical system using a rod integrator for flattening the spatial light intensity distribution of the illumination light has been proposed (for example, refer to U.S. Pat. No. 6,868,223 and a U.S. Pat. No. 6,789,931).
As mentioned above, the brightness and contrast of the projection image, which is projected on a screen, have a so-called trade-off relationship. Therefore, when projecting using the projection apparatus disclosed in Laid-Open Japanese Patent Application Publication No. H05-188345, there was a problem that the brightness fell when the aperture diameter of the variable-aperture diaphragm of the projection optical system was made smaller to obtain high contrast.
Even when the illumination optical system using the rod integrator disclosed in the U.S. Pat. No. 6,868,223 and in the U.S. Pat. No. 6,789,931 is used as an illumination optical system of the projection apparatus to flatten the light intensity distribution, there is a problem that the decrease of the brightness caused by making the aperture diameter of the variable-aperture diaphragm of the projection optical system smaller could not be controlled.
In particular, in the case of the projection apparatus in which the discharge lamp is used as a light source, since the amount of light in the area near to an optical axis was especially small, the above-mentioned problem is particularly significant.
The present invention has been made to solve the above technical problems. An objective of the present invention is to provide a projection apparatus which is provided with the variable-aperture diaphragm and can control the decrease of the brightness at the time of making the aperture diameter of the variable-aperture diaphragm smaller, and which can obtain good projection images with a high contrast.